An Open Data Protocol (OData) primer for developers

An Open Data Protocol (OData) primer for developers

Learn how to explore the data in an OData service, and the functionality included in the service.

You will learn

Throughout this tutorial series you’ve been working on an app that consumes an OData service. Since you are now familiar with using the content, its time to learn a bit more about what the OData protocol can do for a developer. In the next tutorial, you will learn how to apply these capabilities to your SAPUI5 app.

Details

OData (Open Data Protocol) is an OASIS open industry standard covering building and consuming RESTful APIs. The standard was initially created by Microsoft, and the committee is now chaired by a Microsoft and an SAP employee.

There is a wealth of information available at http://www.odata.org, but this tutorial will focus on a few practical aspects that will help you in future projects. The topics covered are:

Browser extension to view JSON easily

OData URI format

Service Document and metadata

Query options:

$format

$top

$skiptoken

$orderby

$filter

$select

$expand

Step 1: Get browser extension

To help visualize an OData feed, it is useful to install a formatting extension in your browser. There are a few options available to select 7#151; the one used in this tutorial is “JSONView” and is available for Chrome, Firefox and Safari browsers. The process for installing an extension is similar across browsers, the Chrome steps are shown below.

In Google Chrome, click the menu button and then select Settings.

Click on Extensions and then at the bottom of the page, click on Get more extensions.

The metadata document will show the individual fields, formats and navigation properties of all the collections. To view the metadata for any OData service, append $metadata at the end of the URL in your browser:

http://services.odata.org/V2/Northwind/Northwind.svc/$metadataWith the metadata displayed, scroll down to <EntityType Name="Product"> which is the collection you are using. You may notice that there is a slight change in the naming. The collection href (or the external name) in step 7 is Products, and in the metadata the same collection’s name is Product. The mapping of an EntityType to a collection name is defined in the EntityContainer elements. You will learn more about the OData model structure in the OData model tutorial.

In your app, you have used the fields displayed including the NavigationProperty entry that points to Suppliers.

In “OData parlance”, a NavigationProperty is a link from an Entry to one or more related Entries. In your app, Web IDE used the link from Products to Suppliers to display some supplier information in the information tab. Specifically, the Supplier CompanyName, Phone and Address fields.

If you want to add other supplier fields to the Supplier tab – you would just need to enter the appropriate Label and Text elements to the DetailInfoForm.fragment.xml file.

Viewing the metadata is a quick way to see what data is available, but it is also useful to view the data itself. To see a set of data from a collection, simply enter the URL to the service document followed by the collection of interest. For the Products collection, it is:

Scroll to the bottom of the page and look for the <link rel="next" entry. The Northwind service enforces server-side paging and will only pass 20 records at a time. The paging size (20) can be seen in the $skiptoken=20 query option at the end of the “next” URL. A Web IDE generated app will automatically issue the “next” URL when you scroll to the bottom of the master list in your app. You can run your app and try this now. Look for the spinning “busy” UI element to appear briefly as the new request is sent and set loaded in.

The XML format of the OData response includes a lot of extra characters that makes the output difficult to read. You can add a query option to the URL to change the format to JSON, and with JSONView installed, some formatting and color-coding makes it more “human-readable”.

After the resource path, multiple query options can be added. Following standard HTTP query string formats, the first option will be pre-pended with a ?, and any successive ones will be pre-pended with a &. To request the response in JSON, use the query option: $format=json pre-pended with a ?.

Some OData services do not enforce server-side paging, and will send a large amount of data for each request. To limit the set of data sent, include the $top=x query option, where x is any integer. $top=1 will request only the first record, $top=5 requests the first five.

Enter $top=1 to view the first record only (be sure to pre-pend it with a & since it is the second query option).

To see the 6th and 7th records, add the $skiptoken=5 query option and change $top to 2. The $skiptoken will make the service skip over the skiptoken many records before it sends data. Changing $top to 2 will return two records, rather than just one.

In the metadata, the ProductID is set as the key for the collection. This means that any data returned will be sorted by ProductID, as evidenced in the screenshots above (ProductID of 1, 6, 7, 22 etc.).

You can specify that the results should be returned sorted on a different field by using the $orderby=<FieldName> query option. It is easier for a person to find a desired product if the list was sorted alphanumerically by ProductName. To see this in action, use the URL below that includes the $orderby=ProductName query option.

By default, the $orderby option sorts in ascending order. To sort by descending order, append “desc” (with the space) after the orderby field. The browser will encode the space as %20 and the results are returned in descending alphanumerical order.

A very useful query option to highlight is the filter expression ($filter). A filter expression can be simple logical operators (equal, greater than, less than, etc.) include basic math (add, subtract, multiply and delete) and functions (string, data, math and type functions), and combinations of all.

A simple example of filtering would be to see which products in the Northwind service are discontinued (the link below will show eight results). The query option string is: $filter=Discontinued eq true

To get a list of products with a UnitPrice greater than 100 and not discontinued, the query option string would be: $filter=Discontinued eq false and UnitPrice gt 100. Here, gt stands for “greater than”.

The $select query option specifies a subset of the full collection properties to return which is useful is you want to minimize the amount of data sent or received to the app. For example, to return only the ProductNam, UnitPrice and Supplier use this URL:

As you have seen, there is quite a bit of capability exposed in an OData service. The advantage to the developer is the application logic that you would otherwise have to create and maintain in code can be delivered by the OData service. You will learn how to do this in the next tutorial.

There are many OData resources available on the web. A few are listed below: